Orchard Soil Fumigation
Pathogenic soil organisms are present in the soils of all mature orchards, and often reduce root growth of young fruit trees when the site is replanted. Poor root development leads to reduced vegetative growth and fair to poor fruit yields throughout the life of the replanted orchard. Certain soil fumigants have controlled the Specific Orchard Replant Disease when properly applied. The positive effect of controlling this disease can be measured 20 years after treatment. No soil treatments will effectively control replant disease problems after planting.
EPA is requiring new safety measures for soil fumigant pesticides (chloropicrin, dazomet, metam sodium/potassium, and methyl bromide). Risk Mitigation Measures for 2010 include reclassification of fumigants to Restricted Use, new Good Agricultural Practices, rate reductions, use site limitations, new handler protections, tarp cutting and removal restrictions, extended worker reentry restrictions, and safety information for handlers. For more detailed information on these safety and regulatory measures, please see the EPA’s webpage, Soil Fumigant Toolbox at http://www.epa.gov/pesticides/reregistration/soil_fumigants.
Restricted Use Pesticide Classification: All soil fumigant products containing methyl bromide, 1,3-dichloropropene, and chloropicrin are currently restricted use pesticides. EPA has reclassified metam sodium/potassium and dazomet as restricted use pesticides. Refer to WSDA's electronic list of soil fumigant products registered in Washington at http://www.kellysolutions.com/WA/showproductssoilfumigants.asp.
Good Agricultural Practices: Older fumigant labels recommend practices that help reduce off-gassing and improve the safety and effectiveness of applications. Revised labels include some of these practices as requirments rather than recommendations to help minimize inhalation and other risks from fumigant applications. Examples of good agricultural practices include proper soil preparation/tilling, ensuring optimal soil moisutre and temperature, appropriate use of sealing techniques, equipment calibration, and weather criteria.
Agricultural Worker Protections: Persons engaged in any of a number of activities that are part of the fumigation process are considered “handlers.” Handler activities include operating fumigation equipment, assisting in the application of the fumigant, monitoring fumigant air concentrations, and installing, repairing, perforating, and removing tarps.
Respiratory Protection: Revised labels in 2010 will require handlers to either stop work or put on respirators if they experience sensory irritation. In most cases inhalation risks can be mitigated with the use of air purifying respirators if they choose to continue working after experiencing sensory irritation. For scenarios involving very high air concentrations where these respirators do not provide adequate protection, EPA is requiring handlers to stop work and leave the area. If there is an emergency, then a self-contained breathing appartus is required for applications involving methyl bromide or chloropicrin.
Tarp Perforation and Removal: Fumigant gases become trapped under tarps and can be released when the tarp is perforated (i.e., cut, punched, poked) and removed (for application methods in which tarps are removed before planting). Handlers perforating and removing tarps may be exposed to air concentrations of concern. To reduce these exposures, EPA is will be requiring a minimum interval of five (5) days between application and tarp perforation; a minimum interval of two (2) hours between perforation and tarp removal; that handlers stop work or use respiratory protection if irritation is detected; and, mechanical devices be used for tarp perforation with few exceptions.
While many soil fumigants, fungicides, fertilizers and other products have been tested for effect on the orchard replant disease, only three active ingredients that make up two choices remain that have shown long-term growth and yield benefits in Washington orchard trials: metam, and fumigants containing 1,3-DCP + chloropicrin.
Most fumigants must be custom applied, others may be applied by a certified private applicator, but current application regulations make it very difficult to do so legally. As fumigant labels and regulations are changing rapidly , pay special attention to use and safety information on a current label. Used improperly, fumigants can be quite hazardous to the applicator and the crop, and will not effectively control orchard replant disease. Some application methods described on fumigant product labels have not resulted in adequate replant disease control.
Follow soil temperature and preparation guidelines on the product label. In general, colder, wetter, compacted and finer textured soils retain fumigants longer. The soil is usually in best condition for fumigation in October and early November. Treatment should be completed well ahead of the time that soil temperatures drop below the minimum recommended on the label. To reduce the chance of fumigant damage to the newly planted trees roots, dig planting holes or disturb the planting area soil a few days prior to planting.
It is far better to plant later than usual in the spring than to risk tree damage by planting while potentially dangerous fumigant residues remain in the soil. Skipping fumigation because it sets back the planting date is a poor choice with long-term negative economic impacts. By October, a May-planted tree planted in fumigated soil will usually out-perform a March planted tree suffering from even a mild case of replant disease. Long-term productivity should be the main concern, not date of planting.
Fumigants are safe and effective when properly used, but special training is required and highly recommended for first time users. Use of other pesticides or fumigants does not qualify as adequate user experience, as each fumigant has unique properties. Before using any fumigant, carefully read and follow safe handling and protective equipment information on the label. Special respirator canisters and vapor-proof eye protection may be required.
Chloropicrin + DCP mixtures (Telone C-17 or 35, other trade names):
Watch for regulation changes that may affect how you can use soil fumigants.
Chloropicrin was the first soil fumigant found to effectively control replant disease. It is most effective controlling soil fungi, and much less effective on nematodes and insects. This product moves no more than 9 to 12 inches from the point of injection, so, to be effective, it must be custom applied by special equipment. A large volume of the future root zone must be treated to assure long-term benefits. Therefore, the application equipment must apply the product in a manner that treats the future tree row in a band at least 8 feet wide and 2 to 3 feet deep.
The chloropicrin is usually mixed with 1,3-dichloropropene. The 1,3 DCP reduces nematodes and important root damaging insects. The chloropicrin is often either 17% or 35% of the mixture, but rate may be adjusted by the custom applicator. The 1,3-D and chloropicrin treatments have controlled orchard replant disease as well or better than the past standard methyl bromide, which is now essentially unavailable.
Contact the custom fumigation applicator well ahead of treatment date, and follow the applicator's directions on soil preparation. Most request a cleared, ripped, and reasonably smooth orchard surface prior to application.
Metam Sodium and Metam Potassium:
Watch for regulation changes that may greatly affect how you may use fumigants.
Originally sold as "Vapam," this product is now also available under several other trade names, including: Metam CLR, K-Pam, Sectagon, and Busan. Metam sodium and metam potassium are water soluble liquids moved by sprinkler irrigation water into the zone of the soil that you wish to treat. After the fumigant and water mixture stops moving downward in the soil, the metam converts into a more toxic fumigant gas. This gas moves only a few inches from the zone treated with the water mixture. Since the active ingredient moves only a short distance, it is critical that the metam and water mix penetrate the future tree root zone 2.5 to 3 feet, but no more. Broadcasting the product with sprinkler irrigation resulted in long-term tree growth and yield improvement. Metam products are sold with various percentages of active ingredient. Apply 100 gallons per acre of those with 33% a.i., and 75 gallons per acre of products containing 42% a.i. Lower rates have resulted in reduced tree growth and yields.
Prior to application soil should be 45° to 75°F and relatively moist (over 85% of field capacity). Pre-irrigate the field if the soil is even moderately dry. Use approximately 1/2 to 1 inch of sprinkler irrigation water to drive the fumigant to the desired depth. Without immediate and continuous incorporation with water, the product will evaporate rapidly, creating a drift and applicator hazard. Over-application of water will over-dilute the product in the soil and greatly reduce the fumigant effect. Sandy, wet, and unworked soils require the lesser rate of water; finer textured, ripped, and drier soils require the higher amount. Measure the irrigation system application rate to determine the hours of irrigation that will apply the proper amount of water. Most systems should be run 3 to 5 hours.
It is not always practical to work the orchard soil prior to treatment. If the soil is prepared for planting after treatment, do not mix untreated soil into the fumigated area.
Metam sodium products have a number of application methods on their labels. The only practical and effective treatment methods of replant disease involve driving the product into the soil with sprinkler irrigation water. Shanking or rototilling the product into the soil or filling planting holes with large volumes of water mixed with a per-site rate of the fumigant has not been as effective.
Fall treatment will allow you to plant the treated site in late winter or early spring. If fall weather or lack of autumn irrigation water delays treatment until spring, label instructions require 3 to 4 weeks between treatment and planting. Soil in the treated area may be prepared for planting starting 10 to 14 days after treatment. Digging planting holes or disturbing the soil a few days prior to planting speeds the release of fumigant residues that may remain. Note: Application variables often lead to variation in disease control.